1. Field of the Invention
The present invention relates to a bridged metallocene complex which can be used for the (co)polymerization of olefins.
More specifically, the present invention relates to a particular bridged metallocene complex of a transition metal, in addition to a catalyst comprising said complex, or deriving therefrom, suitable for the polymerization or copolymerization of ethylene and other .alpha.-olefins, optionally combined with a suitable cocatalyst. The present invention also relates to a method for the preparation of said metallocene complex and the corresponding ligands, as well as a polymerization process of olefins using this.
2. Discussion of the Background
It is generally known in the art that ethylene, or .alpha.-olefins in general, can be polymerized or copolymerized by means of processes at low, medium or high pressure with catalysts based on a transition metal, generally known as catalysts of the Ziegler-Natta type. A particular group of catalysts active in the polymerization of olefins consists of a combination of an organic oxyderivative of aluminum (in particular, polymeric methyl-aluminoxane or MAO) with an .eta..sup.5 -cyclopentadienyl derivative (metallocene) of a transition metal of groups 3 to 6 of the periodic table of elements (in the form approved by IUPAC and published by "CRC Press Inc." in 1989). Particularly interesting results have been obtained with catalysts based on metallocenes of group 4, i.e. which can be defined, in their more general form, by the following formula (I): ##STR2##
wherein M represents a metal of group 4; each R.sub.A independently represents a group of an anionic nature such as, for example, a hydride, a halide, a phosphonate or sulfonate anion, an alkyl or alkoxy group, an aryl or aryloxy group, an amide group, a silyl group, etc.; "w" is an index that can be an integer 1 or 2 depending on whether the valence of M is 3 or 4; Cp represents a ligand of the .eta..sup.5 -cyclopentadienyl type and is generally selected from .eta..sup.5 -cyclopentadienyl, .eta..sup.5 -indenyl, .eta..sup.5 -fluorenyl groups or a substituted derivative of these; R.sub.B may, regardless of the nature of the other substituents, have one of the definitions of either the ligand Cp, or R.sub.A groups. So-called "bridged" metallocenes have also proved to be of particular interest in the known art, wherein two Cp groups, the same or different, are coordinated to the metal M and covalently bound to each other by means of a bivalent organic group. For a known method for the preparation of the above compounds, reference should be made to the description of H. Sinn, W. Kaminsky, in. Adv. Organomet. Chem., vol. 18 (1980), page 99 and U.S. Pat. No. 4,542,199.
These catalysts generally have a high catalytic activity and a certain versatility when applied to the preparation of polyolefins with specific characteristics, especially with respect to the stereochemical control of the polymerization of .alpha.-olefins such as propylene.
The introduction of a "bridged" group, in particular, allows the two pentahapto-coordinated rings (.eta..sup.5) of the cyclopentadienyl ligand to be kept in a stricter reciprocal position than when the bridge is absent. This modification enables the production of polymers with specific characteristics, at times impossible to obtain with non-bridged metallocenes, depending on the catalytic composition and olefin to be polymerized.
It is known that certain "bridged" metallocene catalysts are capable of polymerizing .alpha.-olefins with a high stereospecificity. Whereas the complex (Ind) .sub.2 ZrCl.sub.2 provides a polypropylene with a low isotacticity index [L. Resconi et al. Macromolecules 25, 6814-6817, (1992)], the corresponding catalysts with ethylidene and dimethylsilyl bridges (in the racemic isomeric form) give polypropylene with an isotacticity of 99% and 97% respectively, as described for example in German patents DE 3.743.321 and DE 3.443.087.
In the publication EP-A 310.734, at least two of the above complexes having formula (I) are mixed with each other to obtain a polymer with an enlarged molecular weight distribution (MWD&gt;3) and which is therefore more easily processable in an extruder. "Makromoleculare Chemie", vol. 194 (1993), pages 1745-1755, describes "bridged" complexes supported on inorganic substrates (Al.sub.2 O.sub.3 MgCl.sub.2) and used in the presence of trialkylaluminum AlR.sub.3, instead of MAO, in the polymerization of propylene, whereas in patent application EP-A 418-044 cationic "bridged" complexes are used, which are active in polymerization even without MAO.
Patent and scientific literature on "bridged" catalysts is very broad. The numerous structures studied and claimed are preferably based on Zr and Hf and contain, as pentahapto-coordinated ligands, cyclopentadienyl (Cp), indenyl (Ind) or fluorenyl (Flu) rings, optionally substituted with appropriate groups in certain positions of the molecular skeleton, in order to improve the performance of the catalyst and resulting polymer. For example, W. Spaleck et al., in "Angewandte Chemie, Int. Ed. Eng." vol. 31 (1992), pages 1347-1349, state that the catalyst Me.sub.2 Si(Ind).sub.2 ZrCl.sub.2 allows the production of a polypropylene with a higher molecular weight if a methyl substituent is placed in position 2 on the indenyl ring, whereas, according to "Organometallics", vol. 13 (1994), pages 954-963, a further substitution with a naphthoic group in position 4, also increases the yield to polymer and tacticity index.
Numerous other examples are cited in patent literature, for example in European patent applications EP-A 582.194, EP-A 537.130, EP-A 574.370 and EP-A 581.754.
In spite of the many advantages with respect to the prior known art, represented by the so-called "classical" Ziegler-Natta catalysts, having an intrinsically heterogeneous and multicentric nature, catalysts based on metallocenes also have various disadvantages however, such as, for example, the production of polymers with an average molecular weight which is still insufficient, especially with polymerization processes at high temperatures. In addition, also in the case of metallocenes, it is desirable to further improve the stereoselectivity in the polymerization of .alpha.-olefins with processes at a high temperature and pressure, of about 150-250.degree. C. and 50-100 MPa. It would also be preferable to further increase the activation and polymerization rate provided by the catalytic system in processes characterized by reduced residence times in the reactor.
Another rather unsatisfactory aspect of the above catalysts relates to their behaviour in the copolymerization of ethylene to produce low density polyethylene or olefinic elastomers, again with respect to the difficulty in obtaining copolymers with sufficiently high molecular weights, suitable for their numerous industrial applications. It is known, in fact, that it is necessary to operate with significant quantities of comonomer to insert the desired quantity into the copolymer, with a consequent increase in the rate of the chain transfer reaction, competitive with the polymerization, and the production of unsatisfactory molecular weights. This disadvantage becomes even more critical when operating with polymerization processes at a high temperature in which the chain transfer reaction is already substantial without the comonomer. Not less significant, in this respect, is the quantity of comonomer inserted, as well as the "means" of insertion, referring to the formation of comonomer block sequences, rather than a more desirable statistic distribution.
Although different types of variously substituted .eta..sup.5 -cyclopentadienyl ligands have been studied in detail in the known art in order to overcome the above disadvantages and improve the characteristics according to the specific applications, there are few publications on the influence, in a polymerization process, of groups forming the "bridge" between these ligands, which are basically limited, in practice, to the groups --CH.sub.2 --CH.sub.2 --, --CMe.sub.2 --, and --Si(R.sub.C R.sub.D)-- (being R.sub.C and R.sub.D alkyl or aryl groups).
The publication "Makromolekulare Chemie, Rapid Comm.", Vol. 14 (1993), pages 633-636, describes particular polymerization catalysts based on bis-(.eta..sup.5 -cyclopentadienyl) complexes containing a bridge between the two ligands consisting of a 1,3-phenylenedimethylene group. These complexes, although capable of polymerizing ethylene in the presence of MAO, have poor solubility in aromatic and/or aliphatic hydrocarbons, and a much lower activity than that of the commoner metallocene complexes, such as, for example (.eta..sup.5 -C.sub.5 H.sub.5) .sub.2 ZrCl.sub.2.
The publication "Acta Chimica Sinica", vol. 48 (1990), pages 298-301, describes the preparation of some zirconium and titanium bis-cyclopentadienyl complexes, which contain a phenylenedimethylene bridge between the two cyclopentadienyl ligands. No mention is made, however, in this publication of the possible use of these complexes in the polymerization of olefins.
European patent application EP-A 752.428, filed by the Applicant, discloses bridged metallocene complexes in which the two .eta..sup.5 -cyclopentadienyl groups are bridge-bound with a divalent group having the formula --CH.sub.2 --(A)--CH.sub.2 --, wherein A is a divalent unsaturated hydrocarbon group. Although these complexes allow a reasonable reaction rate to be reached in the formation of olefinic homo- and copolymers, their insertion capacity of the comonomer in the copolymerization of ethylene, is still unsatisfactory.